As I said. On grass you don’t want shadow.
unless you are rendering for a movie or something similar, a game cannot afford the cost of casting shadows on 30k tris as easily as anyone who has never made a game is lead to think.
That said,
Only thing I can think of with scale affecting shadows is DFAO. Since when you have a bigger object, the volume is also bigger, though not more detailed.
The issue could be something else entierly too, I’d start with reviewing the mesh itself, and the vertex normals on it.
then the method of painting - just use the grass node…
Then the scene’s light settings - including the contact shadows setting maybe…